The present invention relates to a method and circuit for correcting an output signal of a sensor, such as an angular velocity sensor or an acceleration sensor.
Sensors have been miniaturized over recent years and have finer output signals. Such a sensor requires an amplification circuit for detecting and amplifying sensor output. Output signals from the sensor and the amplification circuit are dependent on temperature. Thus, the amplification circuit is required to correct such temperature dependent characteristics. Accordingly, there is a need for performing such a correction process accurately and easily.
In the prior art, a digital correction process and an analog correction process have been employed by sensor amplification circuits to correct the temperature dependent characteristic of a sensor output. The digital correction process uses correction data prestored for every predetermined temperature step in a storage means. The amplification circuit reads correction data corresponding to an ambient temperature from the storage means and generates an output signal by correcting the sensor output based on the correction data.
In the analog correction process, when the temperature dependent characteristic of the sensor output has a predetermined gradient, a gradient is set for the temperature characteristic of the amplification circuit to offset the predetermined gradient of the sensor output. When the sensor output changes along a curve with respect to temperature changes, the sensor output is approximated with a plurality of straight lines to switch the gradient of the temperature dependent characteristic of the amplification circuit in accordance with the temperature. In this way, the amplification circuit corrects the sensor output to generate the output signal.
FIG. 1 is a schematic block circuit diagram of a conventional sensor amplification circuit 2 including a digital correction function. The amplification circuit 2 is formed by an IC chip and connected to a bridge type sensor 1. The amplification circuit 2 includes an amplification circuit unit 4 for amplifying the output of the sensor 1 and generating an output signal Vout. The amplification circuit unit 4 includes a digital correction function for correcting the output of the sensor 1 in a digital manner. The sensor 1 is supplied with constant current from a current source 3 in the amplification circuit 2. Output voltages Vs1 and Vs2 of the sensor 1 are supplied to the amplification circuit unit 4.
The amplification circuit unit 4 includes input-stage amplifiers 5a and 5b for receiving the output voltages Vs1 and Vs2 of the sensor 1, an amplifier 6 for amplifying the difference between output voltages of the input-stage amplifiers 5a and 5b, an amplifier 7 for amplifying an output signal of the amplifier 6, and an output-stage amplifier 8 for amplifying an output voltage of the amplifier 7.
The input-stage amplifiers 5a and 5b is respectively connected to the feedback resistors R1a and R1b, which are variable resistors. The resistances of the feedback resistors R1a and R1b are adjusted to adjust the gain of the amplification circuit unit 4. The amplifier 6 includes two input terminals, one of which is connected to ground GND via a voltage adjustment circuit 9. The voltage adjustment circuit 9 has a voltage that is adjusted to adjust the offset voltage of the amplification circuit unit 4.
The amplifier 7 is connected to a feedback resistor R2, which is a variable resistor. The resistance of the feedback resistor R2 is adjusted to adjust the gain of the amplification circuit unit 4. The output-stage amplifier 8 includes two input terminals, one of which is connected to ground GND via a voltage adjustment circuit 10. The voltage adjustment circuit 10 has a voltage that is adjusted to adjust the offset voltage of the amplification circuit unit 4.
Each of the resistors R1a, R1b, and R2 includes a plurality of resistors connected in series and a plurality of switches respectively connected in parallel to the resistors. The switches for each of the resistors R1a, R1b, and R2 are controlled to adjust the resistance of each of the resistors R1a, R1b, and R2 in steps. Each of the voltage adjustment circuits 9 and 10 includes a plurality of resistors connected in series and a plurality of switches respectively connected in parallel to the resistors. The switches for each of the voltage adjustment circuits 9 and 10 are controlled to adjust the voltage in steps for each of the voltage adjustment circuits 9 and 10.
The amplification circuit unit 4 digitally corrects the temperature characteristic of the sensor 1 in accordance with the operation of a control circuit 11. The amplification circuit unit 4 amplifies the output of the sensor 1 to generate an output signal Vout, which is not dependent on the temperature.
The control circuit 11 is connected to a temperature sensor 12 and a memory device 13a. The temperature sensor 12 detects the ambient temperature. The memory device 13a prestores correction data for adjusting the resistances of the resistors R1a, R1b, and R2 and the voltage adjustment circuits 9 and 10 in accordance with the temperature detected by the temperature sensor 12. The control circuit 11 reads correction data corresponding to the ambient temperature from the memory device 13a based on a detection signal from the temperature sensor 12. The memory device 13a converts the correction data into control data and stores the control data in a data latch unit 13b. The resistances of the resistors R1a, R1b, and R2 and the voltage adjustment circuits 9 and 10 are adjusted based on the control data stored in the data latch unit 13b. 
FIG. 2 is a graph schematically showing the output characteristic of the sensor amplification circuit 2. Characteristic curve X1 indicates the output characteristic of the amplification circuit unit 4 when digital correction is not performed. As shown in FIG. 2, the characteristic curve X1 increases as the temperature increases. However, it is preferable that the characteristic curve X1 be corrected in a manner that the temperature characteristic is flat at a predetermined target value (zero level in FIG. 2) regardless of the temperature.
Characteristic curve X2 indicates the output characteristic of the amplification circuit unit 4 after digital correction. As the characteristic curve X2 of FIG. 2 shows, the resistances of the resistors R1a, R1b, and R2 and the voltage adjustment circuits 9 and 10 are adjusted in intervals of 10° C. at correction points P1 to P10.
As shown in FIG. 2, the characteristic curve X2 indicates that the level of the output signal converges on its target value at each of the correction points P1 to P10. However, the characteristic curve X2 deviates from the target value in the temperatures between the correction points P1 to P10 in accordance with the temperature dependent characteristic expressed by the characteristic curve X1. Thus, the characteristic curve X2 is plotted in a sawtooth-like manner depending on the temperature. As a result, the characteristic curve X2 is still temperature dependent and not flat.
FIG. 3 is a schematic circuit diagram of a conventional sensor amplification circuit 20 including an analog correction function. An output-stage amplifier 14 includes two input terminals, one of which is supplied with output voltage Vs of a sensor and the other of which is supplied with output voltage Va of an analog correction amplifier 15.
Temperature dependent voltage Vt, which is temperature dependent as shown in FIG. 4(b), and reference voltage Vref, which is not temperature dependent and constant as shown in FIG. 4(c), are respectively supplied to the two input terminals of the analog correction amplifier 15 via a switch circuit 16. The temperature dependent voltage Vt is generated by a forward voltage at a PN junction of a transistor or a diode. The temperature dependent voltage Vt changes linearly with respect to temperature changes at a gradient of, for example, −2 mV/° C. The reference voltage Vref is generated using a bandgap reference voltage.
The switch circuit 16 supplies the temperature dependent voltage Vt and the reference voltage Vref respectively to the two input terminals of the analog correction amplifier 15. If one of the input terminals of the amplifier 15 is supplied with the temperature dependent voltage Vt, the other one of the input terminals of the amplifier is supplied with the reference voltage Vref. If one of the input terminals of the amplifier 15 is supplied with the reference voltage Vref, the other one of the input terminals of the amplifier is supplied with the temperature dependent voltage Vt.
A feedback resistor R3, which is a variable resistor, is connected between the one of the input terminals and an output terminal of the analog correction amplifier 15. A variable resistor R4 and a voltage adjustment circuit 17 are connected in series between the other one of the input terminals of the analog correction amplifier 15 and ground GND.
The resistors R3 and R4 and the voltage adjustment circuit 17 are each configured in a manner similar to the resistors R1a, R1b, and R2 and the voltage adjustment circuits 9 and 10 shown in FIG. 1. Accordingly, the resistances of the resistors R3 and R4 and the voltage of the voltage adjustment circuit 17 are adjusted in steps.
The output voltage Va of the analog correction amplifier 15 changes linearly based on changes in the ambient temperature as shown in FIG. 4(e). In accordance with changes in the ambient temperature, the switch circuit 16 switches the output voltage Va between the gradient of the solid line shown in FIG. 4 (e) and the gradient of the broken line shown in FIG. 4(e). The gradient of the output voltage Va is changed by adjusting the resistances of the resistors R3 and R4. Further, the offset voltage of the output signal Vout of the output-stage amplifier 14 is changed by adjusting the voltage value of the voltage adjustment circuit 17.
As shown in FIG. 4(a), when the output voltage Vs of the sensor 1 changes linearly with respect to the ambient temperature, the gradient of the output voltage Va of the analog correction amplifier 15 is set to offset the gradient of the output voltage Vs of the sensor 1. As a result, the output signal Vout that is not temperature dependent as shown in FIG. 4(d) is output from the output-stage amplifier 14.
FIG. 5 is a graph schematically showing the output characteristic of the sensor amplification circuit 20 shown in FIG. 3. Characteristic curve X1 shows the output characteristic of the amplifier 14 when analog correction is not performed. The amplification circuit 20 approximates the characteristic curve X1 from two straight lines L1 and L2 and switches the resistances of the resistors R3 and R4 and the voltage adjustment circuit 17 so as to offset the gradients and the offset values of the straight lines L1 and L2. For example, the resistances of the resistors R3 and R4 are switched at 10° C. to switch from the approximate straight line L1 to the approximate straight line L2.
In FIG. 5, the characteristic curve X3 is substantially converged to the target value (level represented by zero in FIG. 5) at temperatures in which the characteristic curve X1 coincides with the approximate straight lines L1 and L2. However, the characteristic curve X3 deviates from the target value in the vicinity of 10° C. at which the switching between the approximate straight lines L1 and L2 occurs. Accordingly, a flat temperature characteristic cannot be obtained.
Japanese Laid-Open Patent Publication No. 2003-84728 describes a voltage generation circuit that includes a circuit for performing temperature compensation through analog control and a circuit for performing temperature compensation through digital control. The voltage generation circuit switches between analog control and digital control in accordance with the temperature region.
Japanese Laid-Open Patent Publication No. 11-64123 describes a bridge circuit that includes a compensation resistor for performing rough compensation for an output that changes in accordance with the temperature and a compensation unit for performing fine compensation for that output.
Japanese Laid-Open Patent Publication No. 11-194061 describes a sensor drive circuit for performing temperature compensation in a sensor drive circuit with a digital compensation means.
Japanese Laid-Open Patent Publication No. 2001-143183 describes a structure similar to the analog correction amplifier 15 shown in FIG. 3.